Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 31, 2021
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Trading Symbol	ROKU
Entity Registrant Name	Roku, Inc.
Entity Current Reporting Status	Yes
Entity Central Index Key	0001428439
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity File Number	001-38211
Entity Tax Identification Number	26-2087865
Entity Address, Address Line One	1155 Coleman Avenue
Entity Address, City or Town	San Jose
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	95110
City Area Code	408
Local Phone Number	556-9040
Entity Incorporation, State or Country Code	DE
Title of 12(b) Security	Class A Common Stock, $0.0001 par value
Security Exchange Name	NASDAQ
Entity Interactive Data Current	Yes
Document Quarterly Report	true
Document Transition Report	false
Class A Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		117,707,415
Class B Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		16,655,344